Erf function using Horner's method

erf.js

//Ported from: http://stackoverflow.com/questions/457408/is-there-an-easily-available-implementation-of-erf-for-python
var erf = function(x) {
    var t = 1/(1+0.5*Math.abs(x));
    var result = 1-t*Math.exp( -x*x -  1.26551223 +
            t * ( 1.00002368 +
            t * ( 0.37409196 +
            t * ( 0.09678418 +
            t * (-0.18628806 +
            t * ( 0.27886807 +
            t * (-1.13520398 +
            t * ( 1.48851587 +
            t * (-0.82215223 +
            t * ( 0.17087277)))))))))
        );
    return x >= 0 ? result : -result;
};